Communication with a smart card requires a session to be opened between the smart card and the application that wishes to communicate with it. The secure nature of a smart card requires that only one session can be open at any given time. Most smart card readers (SCR) are connected to one system at a time, which allows for its exclusive use by the attached system. Many systems take advantage of this exclusivity by maintaining an open session with the smart card for the duration of the time the smart card is inserted in the reader.
With some SCRs, it is possible (even likely) that more than one connection will exist at a given time. However, if a session is opened by one of the connected systems, it is generally not possible for another connected system to open the session for its own use until the system which currently has the open session decides to close its session.
This may be the case when both a handheld device and a PC are connected to the SCR. When the PC is notified by the SCR that a smart card has been inserted, the PC will typically open the smart card session and keep the session open until the connection with the reader is terminated or the smart card is removed from the SCR. Since the smart card session is always being held open by the PC, the handheld device cannot initiate a session with the smart card. This prevents the handheld device user from performing operations such as signing or decrypting emails, and if the smart card is required for authenticating the user for use of the handheld device, the user will not be able to unlock the handheld device.